1. Field of the Invention
The present invention relates to a magnetooptical recording apparatus capable of performing an overwrite operation which writes new information over the former information previously written on a magnetooptical recording medium.
2. Prior Art
A magnetooptical recording medium such as a disc has a magnetic thin film having perpendicular anisotropy on a light transmitting base. FIG. 9 typically illustrates the magnetization of such a perpendicular magnetization film 1A in a non-recorded or erased state, wherein the direction of the magnetization is indicated by arrows. For example, as shown, the film 1A is uniformly magnetized in one direction of its thickness throughout the entire area thereof. FIG. 10 typically illustrates the magnetization of the film 1A in a first recorded state obtained on the basis of first recording information, wherein the magnetization is inverted in a region 2A. Such recording is performed by first irradiating, for example, with a laser beam applied to the magnetizable film 1A to heat the irradiated spot up to its Curie temperature or a recording temperature for a compensation temperature, and then applying an external magnetic field thereto to cause inversion of the magnetization. For rewriting the recorded film 1A by another recording based on second information, it is necessary to erase the first information previously recorded. If such erasure is performed merely by the application of a magnetic field with regard to the entire area of the magnetooptical recording medium, the erasure time required becomes as long as several times ten minutes with increase of power consumption in the case of erasing, for example, a five-inch disc along its recording track by continuously applying a magnetic field thereto.
As compared with the above, in an overwrite mode where a first recorded state shown in FIG. 10 is rewritten to a second recorded state based on second information as shown in FIG. 11, no particular time is needed for erasure and consequently the rewrite can be achieved with a high efficiency.
There is known a technique of magnetic field modulation to perform such overwrite operation, but it has not yet been put to practical use due to various problems. One of the problems resides in that, for obtaining a second recorded state of FIG. 11 in the overwrite operation executed by the general magnetic field modulation, it is necessary to generate, in accordance with an information signal, recording and erasing magnetic fields of positive and negative polarities which are mutually opposite with respect to the direction of thickenss of the film 1A, in such a manner that the magnetization of a recorded region 2A in FIG. 10 is inverted and erased while new inversions of magnetization are caused in other regions 2B and 2C. And for causing such inversions at a high frequency, it is requisite to reduce the number of turns of the electromagnetic coil in the magnetic field generating means so as to minimize the inductance thereof. In this case, however, the current for energizing the coil needs to be greater correspondingly to such reduction, hence raising another problem of generation of heat. Furthermore, a driving power source required to invert such a great current with positive and negative polarities at a high frequency becomes dimensionally large with a considerable amount of power consumption. In addition, it is extremely difficult to attain satisfactory frequency characteristic in the above case.
Meanwhile, FIG. 8 graphically shows the recording characteristic of a magnetooptical recording medium which has a perpendicular magnetization film composed of TbFeCo, measured when an external magnetic field is applied thereto, wherein a broken-line curve represents a recorded noise level and a solid-line curve represents a recorded carrier level. It is obvious therefrom that a high noise level and hence a low C/N are induced in a range of the external magnetic field from H.sub.SI to H.sub.SII or, more specifically, from +100 (Oe) to -100 (Oe) in this example. (Hereinafter the range from H.sub.SI to -H.sub.SII is referred to as high-noise magnetic field region). Accordingly, in erasing and recording this magnetooptical recording medium, application of a proper magnetic field outside of such high-noise magnetic field region is a requisite for raising the C/N, and therefore the overwriting positive and negative field levels for the above-mentioned recording and erasing operations are required to be relatively high. As a result, the magnetic field generating means for applying such external magnetic field needs to produce a great magnetic field to consequently bring about a disadvantage of increased power consumption.